Reliability of Muscle Oxygen Saturation for Evaluating Exercise Intensity and Knee Joint Load Indicators

Objectives: This study aimed to evaluate the reliability of muscle oxygen saturation (SmO₂) and its correlation with variables from an inertial measurement unit (IMU) sensor placed on the knee at different exercise intensities. Methods: Fourteen university athletes participated in the study. Incremental ergospirometry was performed to exhaustion to calculate V’O₂max, determine training zones, heart rate, and workload using the IMU, and analyze muscle metabolism by SmO₂. Results: The analysis revealed significant differences between moderate-to-high-intensity zones (80–89% vs. 50–69%, Δ = 27% of SmO₂; p < 0.001) and high-intensity zones (90–100% vs. 50–79%, Δ = 35% of SmO₂; p < 0.001). SmO₂ values showed moderate reliability at moderate exercise intensities (e.g., ICC 0.744 at 50%) and high variability at higher intensities, with ICC values around 0.577–0.594, and CV% increasing up to 77.7% at 100% intensity, indicating decreasing consistency as exercise intensity increases. SmO₂ significantly decreases with increasing angular velocity (β = −13.9, p < 0.001), while knee joint load only shows significant correlations with SmO₂ in the moderate-to-high-intensity zones (r = 0.569, p = 0.004) and high-intensity zones (r = 0.455, p = 0.012). Conclusions: SmO₂ is a key predictor of performance during maximal incremental exercise, particularly in high-intensity zones. Moreover, SmO₂ has the potential to serve as a physiological marker of the internal load on the muscles surrounding the knee during exercise. The SmO₂ decrease could depend on the angular velocity and impact of the exposed knee during running.